1. Field of the Invention
This invention relates to an opto-magnetic disk apparatus which can be overwritten by magnetic modulation, and more particularly to a recording method by which a track recording density of an opto-magnetic disk apparatus is enhanced.
2. Description of the Prior Art
Opto-magnetic disk apparatus which make use of magnetic modulation are already known. General construction of an exemplary one of conventional opto-magnetic disk apparatus is shown in FIG. 10.
Referring to FIG. 10, the conventional opto-magnetic disk apparatus shown is of the pulse-driven type and includes a semiconductor laser 1 which serves as a light emitting element and is driven by a laser driving pulse signal SL of a predetermined period. Laser light emitted from the semiconductor laser 1 is collimated into parallel rays of light by a collimate lens 2, passes a beam splitter 3 and is focused on a record face Da of a disk D by an objective lens 4. When the laser 1 emits laser light in response to a driving pulse signal SL in this manner so that a laser spot is formed on the record face Da of the disk D, the record face Da is heated to a temperature higher than its Curie point by energy of the laser light so that a pit is formed on the record face Da. A magnetic head 5 is disposed above the disk D and magnetically modulates the pit formed by heat of the laser spot on the record face Da in accordance with a modulation signal SR conforming to record data.
In a reproducing operation, the semiconductor laser 1 is continuously driven so that a spot of laser light scans pits formed on the record face Da of the disk D and the pits are read by a pin photodiode 6. Thus, reproduction of information conforming to a Kerr rotational angle according to modulation of the pits is performed.
FIG. 11 illustrates such recording operation as described above. Referring to FIG. 11, a waveform (A) shows a clock pulse signal S.sub.C ; waveform (B) shows a laser driving pulse signal S.sub.L ; waveform (C) shows a modulation signal S.sub.R for the magnetic head; drawing (D) shows pits formed on the record face Da of the disk D; drawing (E) shows a cross-sectional view of a modulated condition of the record face Da; and waveform (F) shows a temperature variation of the record face Da at a pit P.sub.1.
According to the conventional recording method for an opto-magnetic disk apparatus as shows in FIG. 10, the semiconductor laser 1 is intermittently driven in the period t by the laser driving pulses S.sub.L in synchronism with the clock pulses S.sub.C. If the time for which the laser 1 emits light in response to a laser driving pulse S.sub.L is t.sub.0, then the record face Da of the disk D is heated for a period of time substantially equal to the driving time t.sub.0, and areas thus heated make pits P.sub.1, P.sub.2, P.sub.3, . . . shown in FIG. 11. The thus heated pits P.sub.1, P.sub.2, P.sub.3, . . . are magnetically modulated in response to a modulation signal S.sub.R for the magnetic head shown by the curve (C) of FIG. 11. In the drawing (D) of FIG. 11, the pits P.sub.1 and P.sub.4 modulated in accordance with the value "1" of the modulation signal S.sub.R applied to the magnetic head 5 are indicated by hatching lines while the other pits P.sub.2, P.sub.3 and P.sub.5 modulated in accordance with the value "0" of the modulation signal S.sub.R are indicated without hatching lines.
Magnetic modulation of a pit proceeds such that, as seen from the waveform (F) of FIG. 11 a pit is heated by a laser spot so that the temperature of the record face of the disk rises beyond the Curie point and then a direction of a magnetic field by the magnetic head 5 is recorded when the temperature drops across the Curie point. A demodulation condition when a semiconductor laser in the present day is used and driven by a pulse exhibits such a steep variation as seen from the drawing (E) of FIG. 11.
The recording method of the pulse-driven type shown in FIG. 11 is superior in that magnetic domains of pits can be made short to achieve a considerably high track recording density comparing with other recording methods.
The above described conventional recording method, however, is disadvantageous in that, since each pit records information of only one bit as seen from FIG. 11, further enhancement of the track recording density is severely limited. In particular, in order to further enhance the track recording density with the recording method of FIG. 11, the length of a pit should further be reduced. However, since a pit heated by a laser spot presents such a thermal response as seen from the curve (F) of FIG. 11, if it is taken into consideration that the time t.sub.A until magnetic modulation of a pit after heating thereof is several tens nanoseconds and also a variation of the laser power and cooling rates on the inside and the outside of a disk are taken into consideration, in order to achieve a high S/N ratio, the pulse period must be set to a value higher than the time t.sub.A. Accordingly, there is a limitation in enhancement of the track recording density.
Further, it seems an idea to effect pit edge recording making use of such a steep rising edge of a modulation condition as seen from the curve (F) of FIG. 11. Such pit edge recording is performed by 2-7 modulation based on RLL (Run Length Limited) coding. In particular, a code of "100", "1000" or the like is recorded for each rising edge of modulation. In this instance, however, the length of a pit in the time base direction must necessarily be controlled in accordance with a length of a code. However, with a conventional method wherein the frequency and the phase of laser pulses are fixed, it is very difficult, for the reason that the influence of a thermal characteristic of a record medium is high and also for some other reasons, to freely control the bit length.